MFC r308424, r310636

r308424:
Fix locking in bcm2835_audio driver

- Move all VCHI activity to worker thread: channel methods are called with
    non-sleepable lock held and VCHI uses sleepable lock.

- In worker thread use sx(9) lock instead of mutex(9) for the same reason.

PR:		213801, 205979

r310636:
[rpi] Fix bcm2835_audio locking and samples starvation

Rework general approach to locking and working with audio worker thread:

- Use flags to signal requested worker action
- Fix submitted buffer calculations to avoid samples starvation
- Protect buffer pointers with locks to fix race condition between callback
  and audio worker thread
- Remove unnecessary vchi_service_use
- Do not use lock to serialize VCHI requests since only one thread issues them now
- Fix unloading signaling per hselasky@ suggestion
- Add output to detect inconsistent callback data caused by possible firmware bug
  raspberrypi/firmware#696
- Add stats/debug sysctls to troubleshoot possible bugs

PR:		213687, 205979, 215194